The present invention relates to an apparatus and method for controlling combustion in internal combustion engines, and more particularly, to an apparatus and method for controlling combustion in an internal combustion engine that injects fuel directly into its cylinders and that shifts combustion modes.
In a typical engine, fuel is injected into an intake port by a fuel injection valve to charge the associated combustion chamber with a uniform mixture of fuel and air. In the engine, an air intake passage is opened and closed by a throttle valve, which is operated in cooperation with an acceleration pedal. The opening of the throttle valve adjusts the intake air amount (and ultimately the amount of uniformly mixed air and fuel) that is supplied to the combustion chambers of the engine. This controls engine power.
However, when performing uniform (homogeneous) charge combustion, the throttling action of the throttle valve drastically decreases the pressure in the intake passage. This increases pumping losses and decreases efficiency. Stratified charge combustion solves such problems. In stratified charge combustion, the throttle valve is opened wide, and fuel is supplied directly into each combustion chamber. This delivers a rich, highly combustible air-fuel mixture to the vicinity of the spark plug and enhances ignitability.
Japanese Unexamined Patent Publication No. 5-52145 describes an apparatus that injects fuel directly into the combustion chamber defined in each engine cylinder. The apparatus computes a basic fuel injection amount, which is a theoretical value representing the engine load, from the engine speed and the depression degree of the acceleration pedal. The apparatus compares the basic fuel injection amount with a shift value and shifts the combustion mode between stratified charge combustion and semi-stratified charge combustion.
Stratified charge combustion is performed when the basic fuel injection amount is less than the shift value. When performing stratified charge combustion, fuel is injected into the combustion chamber during a late stage of the compression stroke. The fuel is concentrated around the spark plug and ignited. Semi-stratified charge combustion is performed when the basic fuel injection amount is greater than the shift value. When performing semi-stratified charge combustion, fuel is injected into the combustion chamber during the intake stroke and then during the late stage of the compression stroke. The divided injection decreases the amount of fuel concentrated around the spark plug. In this state, the fuel is ignited.
In a system that obtains the basic fuel injection amount based on the depression degree of the acceleration pedal, the value of the basic fuel injection amount changes drastically when the acceleration pedal is depressed in a sudden manner. However, when the depression degree of the acceleration pedal changes suddenly, the amount of air drawn into the combustion chamber does not increase in correspondence with the sudden fluctuation of the basic fuel injection amount. Thus, injection of fuel corresponding to the basic fuel injection amount, which is computed from the depression degree of the acceleration pedal, during rapid acceleration or deceleration of the engine, may cause the air-fuel mixture to become rich or lean.
Therefore, in the prior art, the basic injection fuel amount is graded to vary gradually when the depression degree of the acceleration pedal changes suddenly. That is, a graded fuel injection amount is obtained. An amount of fuel corresponding to the graded fuel injection amount is injected from the fuel injection valve. Therefore, the amount of fuel injected from the fuel injection valve optimally corresponds with the delayed increase in the amount of intake air. This maintains the air-fuel ratio at an optimal value.
However, in the apparatus that shifts combustion modes, the basic fuel injection amount is referred to when determining the combustion mode. Thus, if the depression degree of the acceleration pedal changes suddenly when the basic fuel injection amount is close to the shift value, the basic fuel injection amount may move to the other side of the shift value even if the graded fuel injection amount does not. This shifts the combustion mode to a mode that is inappropriate for the actual conditions. As a result, the amount of injected fuel may be inappropriate for the amount of available air. Such excessive or insufficient fuel injection amount leads to engine power fluctuations or misfires in the engine.